1. Field of the Invention
The present invention relates to a non-resonant knock sensor that is mounted on a combustion engine and, when knocking vibration occurs in the engine, converts the vibration into an electric signal by means of a piezoelectric element clamped in the sensor so as to lead the signal outside as an output signal.
2. Description of the Background Art
A conventional knock sensor will be described referring to the attached drawings.
FIG. 7 is a cross sectional view showing the interior construction of a knock sensor 40 generally known by, for example, Japanese Laid-Open Patent Publication No. 2005-337858. The sensor includes a metal base 2 having a discoid flange section 2a, a cylinder section 2b extending from the flange section 2a in its axial direction and a through-hole passing through both the flange section 2a and the cylinder section 2b. An annular lower insulation sheet 3, a lower terminal plate 4, an annular piezoelectric element 5, an upper terminal plate 6, an annular upper insulation sheet 7, an annular weight 8, and a disc spring 9 are inserted onto the metal base 2, in that order from the flange side of the base. A nut 10 is then screwed onto a male screw 2d threaded on the circumference of one end of the cylinder section 2b, so that the above-described annular constituent parts for the knock sensor are fastened onto the flange section 2a with predetermined torque using a tool such as a torque-wrench-tightening head. Thereafter, terminals 11 are connected to the lower terminal plate 4 and the upper terminal plate 6 by soldering, resistance welding or the like. While portions of the base 2 excluding the inner circumferential face and both end faces of the cylinder section 2b are covered with a first cylindrical mold section 12a, a connector section 12b for outputting a signal is made protruded from a side of the first cylindrical mold section 12a, and then integrally injection-molded therewith at one time, so as to form a resin mold 12; the knock sensor is constructed in this way.
A knock sensor of this kind is mounted on a combustion engine with a bolt, not shown in the figure, that is inserted into the through-hole 2c provided in the base 2 in its axial direction. When knocking vibration occurs in the combustion engine, the annular constituent parts for the knock sensor, including the annular piezoelectric element 5 and the annular weight 8, vibrate coupled with the knocking vibration, and the annular piezoelectric element 5 converts this vibration into an electric signal, so that a detected signal is outputted outside through the lower terminal plate 4 and the upper terminal plate 6.
FIG. 8 and FIG. 9 are external views showing a general knock sensor; FIG. 8 is a top view, FIG. 9, a side view, and FIG. 10, a cross sectional view along the line D-D′ in FIG. 9. An injection gate 13 for the resin mold 12 of the knock sensor, as shown in FIG. 8 and FIG. 9, is generally located on the circumference of the first cylindrical mold section 12a, in a position 180° opposing the connector section 12b. As shown by arrows in FIG. 10, mold material molten by injection-molding is injected to branch into two paths around the circumference of the cylinder section 2b of the base 2, and then meets each other at the junction 15 indicated by X. The molten mold material, after meeting each other at the junction 15 indicated by X, continues to fill the connector section 12b. The molten mold material, after having completely filled the connector section 12b, is pressurized by applying pressure by an injection mold machine; thereby, the inherent strength of resin mold can be secured. A junction where molten mold material meets each other after branching into two paths is generally referred to as a weld, in which junction strength of the material (hereinafter referred to as weld strength) is known to be weaker than that in a non-weld portion.
In the conventional knock sensor 40 constructed described above, there arises a time difference until the molten mold material completely fills the connector section 12b of the resin mold 12 after it has met each other at the junction 15 of mold material indicated by X; thereby, after the molten mold material has completely filled the connector section 12b, cooling of the molten mold material at the junction 15 where mold material meets each other progresses due to this time difference, and its hardening progresses accordingly. Therefore, after the molten mold material has completely filled the connector section 12b, the resin strength, even if the material is pressurized by the injection mold machine, remains weaker at the junction 15 (weld portion) of the molten mold material than other portions (non-weld portions).
Meanwhile, the knock sensor is mounted on an engine block, which therefore undergoes heat shock due to heat generated when the engine runs and thermal difference generated between when the engine runs and when it stops. Resin mold for the knock sensor repeats contraction and expansion due to the heat shock described above, which results in the strength of the resin mold deteriorating, so that eventually cracks occur in the weld portion whose resin strength is weaker than other portions.
Cracks that merely occur in the resin mold do not affect output from the knock sensor. However, because the knock sensor is mounted on the engine block so as to be always exposed to moisture and dust, if a cracked portion of the resin mold gets wet, moisture will eventually penetrate into the interior of the knock sensor to reach the annular piezoelectric element 5, resulting in its output failure.
In injection-molding of resin mold for the conventional knock sensor, it is easy-to-manufacture, from a construction point of view, to inject material in a direction perpendicular to the cylinder section 2b of the base 2. However, a weld 16 as indicated by the broken line in FIG. 8 can not be avoided from being formed in a position 180° opposing the injection gate 13.
Since stress in the connector section 12b is generated due to thermal contraction and expansion, in the directions indicated by the thick allows in FIG. 8, thermal stress generated in the connector section 12b is greater than that generated in the first cylindrical mold section 12a of the cylinder section. In the conventional knock sensor, since the injection gate 13 is located in a position 180° opposing the protruding connector section 12b so that the weld 16 is formed between the cylinder section 2b of the base 2 and the connector section 12b, the weld would be in a particularly disadvantageous position.